1. Field of the Invention
The present invention relates to a device for protecting a voltage source and a load supplied with power by said source.
In particular, although not exclusively, it may be applied to power distribution systems in which it is difficult, or even impossible to control the bus and load impedance characteristics. More generally, it applies to all systems that use a direct current, such as in automobiles (42V), telecommunications (48 V), spacecraft, in particular, the international space station ISS.
2. Description of the Prior Art
Protection circuits have already been proposed, but generally such circuits perform a specific function. For instance, current limiting circuits or surge protection circuits, power limiting circuits, current peak suppressing circuits, and overvoltage protection circuits are known.
Surge protection circuits are generally comprised of a switching element such as a MOSFET (Metal Oxide Semiconductor Field-Effect Transistor) combined with a current measuring device. When the current intensity reaches a certain threshold value, the switching element is controlled so as to maintain the current intensity at or below this threshold value, thereby inducing a potential difference across the switching element and therefore, a reduction in the voltage and thus, in the current applied to the load. For that purpose, the switching element must be placed in a substantially linear mode of operation (as opposed to a saturated mode when it is used as a switch-only element). As a result, it has to dissipate power proportionately to the product of the current and the voltage applied thereto. However, the switching element's capacity to dissipate power is limited, and the switching element has to be quickly turned-off to avoid being damaged.
Thus, the current limiting capacity of current limiters is always associated with the maximum time during which they can operate in the linear mode, which is a short duration on the order of a few milliseconds.
In a complex power distribution system, the loads may be of any kind and the load controlling switch may be remote from both the source and the load (this is the case in the international space station). If the switch is used as a relay in the off or on state, such a control poses no stability problem. On the other hand, if the switch is used in a linear mode of operation for controlling the maximum current (as is the case of a current limiter), serious stability problems may occur. Generally, such stability problems are solved by limiting the bandwidth of the current loop, which increases the response time, or by inserting a known and controlled impedance upstream and downstream the current limiter, which requires capacitors and damping networks, thus increasing the required size of such a device.
Finally, such a surge protection circuit needs to be protected against overvoltages which may damage it because the circuit has been triggered in the off-state to protect the load (off-state circuit protection).
The most efficient surge protection circuits (adapted to loads with no overvoltage allowance) are those which short-circuit the voltage source by means of a thyristor, and thus transform the overvoltage into an overcurrent. Such circuits therefore require an overcurrent protection device which has the above-mentioned drawbacks.
Furthermore, this protection is well-suited to fault-induced overvoltages. On the other hand, in complex power distribution systems, these overvoltages may occur in a normal situation and therefore, may not result in the protection circuit being triggered.
Circuits for suppressing voltage peaks generally comprise an RC damping network or Zener diodes or also so-called “transorb” diodes that can absorb an amount of energy by an avalanche effect, and thus, restrict the voltage with a certain accuracy, on the order of +/−10% of their Zener voltage.
These circuits must in any case absorb the peak energy and reduce the voltage down to a safety level, but should also be compatible with overvoltages and transient phenomena liable to occur in the system to be protected. It has been found that in complex electrical power distribution systems, it is nearly impossible to ensure both of these functions through conventional means in a reliable manner.
Power limiting circuits are designed for measuring the voltage and current applied to a load, and control a power supply switching element (such as for current limiters) so as to maintain the product of voltage and current constant. Carrying out this regulation as a function of the product of voltage and current proves to be a complex operation.
Circuits for damping the quality factor (Q factor) have also been suggested. When the impedance of the circuit comprising the power supply bus and the load has a high Q factor, oscillations that occur upon transient overvoltages at the source or transient overcurrents at the load, may be observed. In order to suppress such oscillations, it is known to use a large damping capacitance which is unavoidably bulky and heavy. Such a capacitance is therefore not suited to spacecraft.
Generally, the prior art circuits may not withstand certain overvoltages smaller than those which trigger them upon shut-off. In addition, their operation depends on the source characteristics and the load impedance.